The present invention relates to microelectromechanical structures, and more particularly to temperature compensated thermally actuated microelectromechanical structures and associated methods.
Microelectromechanical structures (MEMS) and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Many different varieties of MEMS devices have been created, including microgears, micromotors, and other micromachined devices that are capable of motion or applying force. These MEMS devices can be employed in a variety of applications including but not limited to hydraulic applications in which MEMS pumps or valves are utilized and optical applications which include MEMS light valves and shutters.
MEMS devices have relied upon various techniques to provide the force necessary to cause the desired motion within these microstructures. For example, cantilevers have been employed to apply mechanical force in order to rotate micromachined springs and gears. In addition, some micromotors are driven by electromagnetic fields, while other micromachined structures are activated by piezoelectric or electrostatic forces. Recently, MEMS devices that are actuated by the controlled thermal expansion of an actuator or other MEMS component have been developed. For example, U.S. patent application Ser. Nos. 08/767,192; 08/936,598, and 08/965,277 which are assigned to MCNC, the assignee of the present invention, describe various types of thermally actuated MEMS devices. The contents of each of these applications are hereby incorporated by reference herein. Thermal actuators as described in these applications comprise arched beams formed from silicon or metallic materials that further arch or otherwise deflect when heated, thereby creating motive force. These applications also describe various types of direct and indirect heating mechanisms for heating the beams to cause further arching, such that the thermal actuator structures move relative to other microelectronic structures when thermally actuated.
In practically every application of MEMS devices, precisely controlled and reliable movement is required. Given the micron scale dimensions associated with MEMS structures, stable and predictable movement characteristics are critically important. The movement characteristics of MEMS devices can be affected by intrinsic factors, such as the type of materials used to fabricate the MEMS device, the dimensions and structure of the MEMS device, and the effects of semiconductor process variations. All of these intrinsic factors can be controlled to some extent by the MEMS design engineer. In addition, movement characteristics may be affected by extrinsic factors such as fluctuations in the ambient temperature in which the MEMS device operates, which cannot be controlled by the MEMS design engineer. While all of the above factors affect the ability of a MEMS device to move precisely and predictably, the impact of these factors may vary from device to device. For instance, while thermally actuated MEMS devices are affected by all the above factors, they are particularly sensitive to ambient operating temperature variations because they are thermally driven devices.
More particularly, a thermally actuated MEMS device may operate unpredictably or erroneously since the MEMS device will move not only in response to thermal actuation caused by active heating or cooling, but also due to changes in the ambient operating temperature. If ambient temperatures are very high, parts of a MEMS device designed to move in response to thermal actuation may move too much or too far. Alternatively, in very low ambient temperatures, parts of a thermally actuated MEMS device designed to move may not move sufficiently in response to thermal actuation thereof. In either temperature extreme, maintaining parts of MEMS device in predictable positions relative to each other can be difficult. Ambient temperature effects can thus affect the reliability and limit the possible applications of MEMS thermally actuated devices. Those skilled in the art will appreciate that similar problems can arise due to residual stress created by semiconductor process variations and structural differences within MEMS devices.
Therefore, while some thermally activated MEMS structures have been developed, it would still be advantageous to develop other types of thermally actuated structures that would operate more reliably or more precisely even when exposed to significant ambient temperature fluctuations. Consequently, these MEMS structures would be suitable for a wider variety of applications. Numerous applications including but not limited to switches, relays, variable capacitors, variable resistors, valves, pumps, optical mirror arrays, and electromagnetic attenuators would be better served by MEMS structures with these attributes.
The present invention provides temperature compensated microelectromechanical structures and related methods that satisfy at least some of the above needs and provide several advantageous features. According to the present invention, the temperature compensated MEMS structures include an active microactuator and a temperature compensation element such as a temperature compensation microactuator or a frame. The active microactuator and the respective temperature compensation elements are adapted to move in unison in response to thermal actuation caused by ambient temperature changes. However, the active microactuator is also adapted to move in response to the active alteration of the temperature of the active microactuator, such as in response to active heating of the active microactuator. Since the active microactuator and the respective temperature compensation element are adapted to move cooperatively in response to changes in ambient temperature, a predefined spatial or positional relationship can be substantially maintained over a range of ambient temperatures in the absence of active alteration of the temperature of the active microactuator. For example, a predefined spatial relationship can be maintained between the active and temperature compensation microactuators as the ambient temperature changes. In an alternative embodiment that includes a frame, a portion of the active microactuator can be maintained in the same relative position with respect to an underlying microelectronic substrate as the ambient temperature changes.
Accordingly, the temperature compensated microelectromechanical structures of the present invention can isolate the movement of the active microactuator that is driven by active alteration of the temperature of the active microactuator from that which is caused by changes in ambient temperature. As such, the temperature compensated microelectromechanical structures of the present invention can operate more precisely than conventional microelectromechanical structures since the effects of ambient temperature changes are eliminated.
In one embodiment, the temperature compensated microelectromechanical structure includes a microelectronic substrate having a first major surface, an active microactuator, and a temperature compensation microactuator. While the temperature compensated microelectromechanical structure can include a variety of types of actuators, the active microactuator and the temperature compensation microactuator are preferably thermal arched beam actuators. Regardless of the type of actuator, the temperature compensation microactuator is disposed upon the first major surface of the microelectronic substrate and adapted for thermal actuation. In particular, the temperature compensation microactuator is adapted to controllably move in response to changes in ambient temperature. Similarly, the active microactuator is disposed upon the first major surface of the microelectronic substrate and adapted thermal actuation. In contrast to the temperature compensation microactuator, however, the active microactuator is adapted to controllably move in response to the cumulative effect of changes in ambient temperature and active alteration of the temperature of the active microactuator. Accordingly, both microactuators are adapted to move in unison in response to changes in ambient temperature so as to compensate for the effects of ambient temperature changes. In particular, the active and temperature compensation microactuators are adapted to move and maintain a predefined spatial relationship relative to each other over a range of ambient temperatures, in the absence of active alteration of the temperature of the microactuators.
In one embodiment, the temperature compensation microactuator comprises a first member and the active microactuator comprises a second member, with each member adapted to move with the respective microactuator. The first and second members can be selectively brought into contact with each other in response to the active alteration of the temperature of the active microactuator, such that the temperature compensation structure of this embodiment can serve as a relay, a switch or the like. As a result of its unique construction, however, the first and second members will not be brought into contact merely by changes in the ambient temperature since both the temperature compensation microactuator and the active microactuator will move cooperatively in equal amounts. The temperature compensation structure of this embodiment can also include a spring adapted to flex and absorb mechanical stresses as the first member and second member are selectively brought into contact.
In a further embodiment, the active and temperature compensation microactuators can include complimentary male and female electrical contacts. The electrical contacts are adapted to move with the corresponding microactuator and are selectively brought into contact with each other in response to the active alteration of the temperature of the active microactuator. In another embodiment, the active microactuator comprises at least one shorting electrical contact, and the temperature compensation microactuator comprises at least two electrically disconnected electrical contacts. In response to active alteration of the temperature of the active microactuator, the shorting electrical contact is moved so as to selectively electrically connect the electrical contacts carried by the temperature compensation microactuator.
Another embodiment of the temperature compensated microelectromechanical structure comprises a microelectronic substrate having a first major surface, a frame disposed upon the first major surface, and an active microactuator also disposed upon the first major surface. The active microactuator is also operably connected to the frame and, in some embodiments, may be disposed within or surrounded by the frame. The frame is adapted for thermal actuation so as to move in response to changes in ambient temperature. In contrast, the active microactuator is adapted to move in response to active alteration of the temperature of the active microactuator. The frame and active microactuator are adapted to move cooperatively in response to changes in ambient temperature such that at least a portion of the active microactuator is maintained in substantially the same relative position with respect to the microelectronic substrate as the ambient temperature changes so long as the temperature of the active microactuator is not actively altered.
The various embodiments of the temperature compensated microelectromechanical structures can be employed in a variety of applications. For instance, the temperature compensated microelectromechanical structures can serve as switches, relays, variable capacitors, and variable resistors. The temperature compensated microelectromechanical structures can also serve as valves, moveable mirrors with or without latches, and electromagnetic radiation attenuators. In addition, the present invention provides related compensation methods for counteracting the effects of ambient temperature variations within microelectromechanical structures.
Regardless of the application, the temperature compensated microelectromechanical structures of the present invention can isolate the movement of the active microactuator that is driven by active alteration of the temperature of the active microactuator from that which is caused by changes in ambient temperature. As such, the temperature compensated microelectromechanical structures of the present invention operate more precisely and predictably than conventional microelectromechanical structures by eliminating the adverse effects of ambient temperature changes.
In order to more precisely define the size of the gap that is part of a MEMS structure, a method of overplating selected surfaces of a MEMS structure is also provided according to the present invention. In this regard, a MEMS structure that defines a gap, such as the contact separation gap between a pair of contacts of a MEMS relay or switch or the separation gap between the conductive plates of a MEMS capacitor, is placed in a plating bath that includes a plating material. Typically, the plating material is a conductive material, such as gold, rhodium, silver, rhuthenium, palladium, or alloys thereof. In addition, other elements or alloys thereof used by those skilled in the art to form electrical contacts may be used as the plating material. Thereafter, the plating material is electroplated onto at least those portions of the MEMS structure that define the gap. By controlling the electroplating process, the resulting gap defined by the overplated portions of the MEMS structure has a predefined size. According to this embodiment, for example, the electroplating process can be controlled such that the resulting gap can be defined to within approximately 0.5 microns, thereby producing a MEMS structure having increased performance characteristics relative to conventional MEMS structures.
Although the foregoing invention will be described in some detail, it will be obvious that certain changes and modifications may be practiced within the scope of the invention described herein.